The catalytic fluorination reactions of hexachloroethane, formed in situ by the addition of chlorine to tetrachloroethylene have long been known.
A currently used process consists of performing these reactions in a liquid phase, in the presence of an antimony halide catalyst. This process easily yields trichloro-1,1,2-trifluoro-1,2,2-ethane, but obtaining dichloro-1,2-tetrafluoro-1,1,2,2-ethane requires high reaction temperatures which involves considerable corrosion of the equipment. Obtaining chloro-1-pentafluoro-1,1,2,2,2-ethane in significant quantities is practically impossible. Moreover, the activity of the catalysts, used in the liquid phase, is very sensitive to the impurities of the reagents, involving extended purifications and raising the cost of the process. Small quantities of pentahaloethanes, which are difficult to separate, are also formed.
Other processes also achieve these gaseous-phase fluorination reactions on various catalysts, such as the oxides or halides of chromium, aluminum, cobalt, iron, titanium, nickel, copper, palladium or zirconium, at temperatures on the order of 200.degree. to 450.degree. C.
These gaseous-phase processes also have a number of disadvantages. The primary disadvantage is the formation, more important in the gaseous phase than in the liquid phase, of trichloro-1,1,1-trifluoro-2,2,2-ethane and dichloro-1,1-tetrafluoro-1,2,2,2-ethane. These isomers, called asymmetric, of trichloro-1,1,2-trifluoro-1,2,2-ethane and dichloro-1,2-tetrafluoro-1,1,2,2-ethane, are undesirable since they are more reactive, and consequently more unstable, than the so-called symmetric isomers. It is acknowledged that for many commercial applications trichlorotrifluoroethane should contain less than 2% of the trichloro-1,1,1-trifluoro-2,2,2-ethane isomer; also, the dichlorotetrafluoroethane should contain less than 15%, and preferably less than 7% of the dichloro-1,1-tetrafluoro-1,2,2,2-ethane asymmetric isomer.
Another disadvantage of the standard gaseous-phase processes is the difficulty of controlling the reaction temperature, because of the strong exothermicity of the reaction of addition of chlorine on tetrachloroethylene.
A third disadvantage results from the deactivation of the catalyst, caused by the polymerization of tetrachloroethylene on the catalytic surface.
Finally, it is difficult to obtain a predetermined distribution of trichlorotrifluoroethane, dichlorotetrafluoroethane and monochloropentafluoroethane with the known gaseous-phase fluorination processes.
To partially relieve these disadvantages, various solutions have been recommended. Thus, U.S. Pat. No. 3,632,834 teaches that the use of a specific catalyst with a chromium trifluoride base in the gaseous phase yields trichlorotrifluoroethane containing not more than about 2% of the trichloro-1,1,1-trifluoro-2,2,2-ethane isomer and dichlorotetrafluoroethane containing not more than about 15% of the dichloro-1,1-tetrafluoro-1,2,2,2 ethane isomer. But this process does not yield chloropentafluoroethane and the transformation rate of hydrofluoric acid does not exceed 62%.
U.S. Pat. No. 3,157,707 describes a process of preparing tetrachlorodifluoroethane, trichlorotrifluoroethane, dichlorotetrafluoroethane and chloropentafluoroethane by passage, in a gaseous phase, of a mixture of tetrachloroethylene, chlorine and hydrofluoric acid over a catalyst having a Cr.sub.2 O.sub.3 base. The asymmetric isomers are produced in small, imprecise quantities. A particular embodiment of this process consists of, first, having the chlorine and tetrachloroethylene react on a Cr.sub.2 O.sub.3 --based catalyst at a temperature of 200.degree. to 350.degree. C., then mixing the effluents with the hydrofluoric acid before sending them into a second zone of chromium oxide catalysis, reaching a temperature of 300.degree. C. This variation only yields trichlorotrifluoroethane and dichlorotetrafluoroethane.
In order to obtain trichlorotrifluoroethane, dichlorotetrafluoroethane and chloropentafluoroethane in controlled proportions, Japanese Patent Application No. 73.26729 recommends a combination of liquid phase and gaseous phase reactions. In a first stage, tetrachloroethylene, chlorine and hydrofluoric acid are made to react in the liquid phase under pressure over a catalyst having an antimony trichloride base, and the raw trichlorotrifluoroethane formed is isolated. The latter is then sent in the gaseous phase in the presence of hydrofluoric acid over an aluminum trifluoride-based catalyst. At 320.degree. C. under 5 atmospheres, 60% of the trichlorotrifluoroethane is converted into dichlorotetrafluoroethane. If the reaction is conducted at 440.degree. C. under 5 atmospheres, a practically quantitative yield of chloropentafluoroethane is obtained. The principal disadvantage of this process is a poor utilization of the heat in the reaction between chlorine and tetrachloroethylene and the relatively large quantities of asymmetric isomers of trichlorotrifluoroethane and dichlorotetrafluoroethane obtained during the gaseous-phase fluorination.
U.S. Pat. No. 118,221 of the German Democratic Republic describes the gaseous-phase preparation of chloropentafluoroethane from a mixture of tetrachloroethylene, chlorine andd hydrofluoric acid. This mixture is first sent to a first catalysis zone which is heated at 350.degree. C. and contains an aluminum trifluoride catalyst doped with nickel fluoride; then, the effluents pass into a second catalysis zone, containing a chromium oxide catalyst heated at 350.degree. C. Trichlorotrifluoroethane, dichlorotetrafluoroethane and hexafluoroethane are formed as by-products. The major disadvantages of this process are the use of two different catalysts, no easy control over the proportions of the products formed, and very high amounts of C.sub.2 Cl.sub.3 F.sub.3 and C.sub.2 Cl.sub.2 F.sub.4 asymmetric isomers.
The applicants' French Pat. No. 1,453,510 teaches a process for purification of dichlorotetrafluoroethane to enrich the symmetric isomer, which consists of having the isomer mixture, possibly in the presence of hydrofluoric acid and/or chlorine, pass, in a gaseous or liquid state, at temperatures between 50.degree. and 500.degree. C., over a catalyst chosen from active carbon, activated alumina, molecular sieves, aluminum fluoride, salts of chromium, cobalt, aluminum, copper, iron or molybdenum, whether or not deposited on a support such as active carbon or alumina. When it is applied in the presence of hydrofluoric acid, this process causes two distinct reactions--a fluorination reaction of dichlorotetrafluoroethane, yielding chloropentafluoroethane, C.sub.2 Cl.sub.2 F.sub.4 +HF.fwdarw.C.sub.2 ClF.sub.5 +HCl, with accessory formation of very small amounts of hexafluoroethane; and a dismutation reaction of dichlorotetrafluoroethane, yielding chloropentafluoroethane and trichlorotrifluoroethane, 2C.sub.2 Cl.sub.2 F.sub.4 .fwdarw.C.sub.2 ClF.sub.5 +C.sub.2 Cl.sub.3 F.sub.3. Since the dichlorotetrafluoroethane asymmetric isomer in these two reactions is more reactive than the symmetric isomer, the result is an enrichment of the untransformed dichlorotetrafluoroethane into a symmetric isomer.